Optical communications systems using a light source and photodetector or signal regenerator optically coupled to each other by means of an optical fiber have reached a stage of significant commercial importance and technical sophistication. Data rates in excess of 10 Gbit/sec and transmission distances in excess of 1000 kilometers are routinely achieved in commercial practice. To reach these levels of importance and sophistication, many types of components have been developed.
To cover a long transmission distance, initial optical communications systems detected optical signals and used these signals to create electrical signals and generates new optical signals using the electrical signals in a repeater. Newer optical communications systems use optical amplifiers to regenerate the signal. In such systems, the optical signals are optically amplified rather than being converted first into an electrical signal. The optical amplifiers are typically rare earth, such as erbium, doped optical fibers. See, for example, IEEE Photonics Technology Letters, pp. 727-729, August 1991, for a description of an erbium doped fiber preamplifier. The preamplifier described in this paper also has a tunable optical filter which the authors stated could have any of several forms including a fiber Fabry-Perot. An exemplary tunable Fabry-Perot filter using optical fibers was described by Miller at the European Conference on Optical Communication, Sept. 16-20, 1990. Use of fibers permitted the size of the filter to be reduced as compared to the size of filters using bulk components. Wavelength tuning was obtained by temperature variations. Numerous uses for the filter were mentioned.
To increase the capacity of communications systems, wavelength division multiplexing systems have been developed. Such systems transmit signals on a plurality of wavelengths. At the receiver, it is frequently desirable or necessary to select a particular wavelength signal from a group of several wavelengths. Wavelength selection may be done by adding a signature to each signal transmitted on the optical fiber. The signature may consist of, for example, a specific tone signal associated with each wavelength. The tone may take the form of a small (amplitude, frequency, and so forth) sine wave superimposed on the already modulated carrier. At the receiver, the characteristic signatures allow identification and locking of the desired wavelength.
Wavelength calibration is typically performed by comparing a reference signal (passive or active) to the transmitted or received signals. The active reference may consist of a very stable source such as a helium-neon or krypton laser while a passive reference may utilize the absorption resonances of a substance such as acetylene gas. Other conventional techniques based on coherent beat or diffraction gratings are known.
The above techniques require constant access to the wavelength reference signal and this requirement reduces system efficiency.